Microwave spectroscopy

ABSTRACT

In a Lide type of absorption cell for use in Stark modulation microwave spectroscopy the cell is in two mutually insulated halves which are secured together by longitudinally spaced releasable clamps allowing the cell to be taken apart readily for cleaning but holding the halves accurately in their correct relative positions when assembled. The clamps can be of C-shape with disengageable leaf springs fitting in their open sides.

United States Patent [1 1 Cuthbert et a1.

[ Jan. 8, 1974 I 1 MICROWAVE SPECTROSCOPY [75] Inventors: John Cuthbert, Bottisham; James Albert Stow, Ely, both of England [73] Assignee: Cambridge Scientific Instruments Limited, Cambridge, England 221 Filed: Jan. 7, 1972 21 App1.N0.: 216,207

[30] Foreign Application Priority Data Jan. 12. 1971 Great Britain 1,343/71 [52] US. Cl 333/95, 356/74, 356/86 [51] Int. Cl. 1101 3/12 [58] Field of Search 333/95; 356/74, 86

[56] References Cited UNITED STATES PATENTS 3.315.186 4/1967 Rosler et a1 333/95 2,155,508 4/1939 Schelkunoff 333/95 2,381,367 8/1945 Quayle 333/95 2,433,368 12/1947 Johnson et a1 333/95 FOREIGN PATENTS OR APPLICATIONS 1,515,958 9/1965 Germany 333/95 883,460 7/1969 Germany 333/95 Primary ExaminerRud0lph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Att0rney-Samuel Scrivener, Jr. et al.

[57] ABSTRACT In a Lide type of absorption cell for use in Stark modulation microwave spectroscopy the cell is in two mu tually insulated halves which are secured together by longitudinally spaced releasable clamps allowing the cell to be taken apart readily for cleaning but holding the halves accurately in their correct relative positions when assembled. The clamps can be of C-shape with disengageable leaf springs fitting in their open sides.

9 Claims, 3 Drawing Figures IIIIIIIIII PATENTED 19 3.784.938

SHEET 2 OF 2 VIJIIIIIIIIII MICROWAVE SPECTROSCOPY This invention relates to the construction of the absorption cell used in the microwave spectroscopy of gases. Such a cell takes the form of a waveguide of rectangular cross-section, usually a metre or up to several metres in length, which contains the gas under examination and a microwave source feeds in power at one end while a detector at the other end examines the resulting absorption spectra.

In the Stark modulation spectrometer an intermittent transverse electric field is applied to the gas in the absorption cell to shift the resonant frequency of the gas molecules by the Stark effect and this field, which must be of the order of at least hundreds of volts per centimetre to achieve an adequate frequency shift, is applied in the known arrangements by means of a flat strip electrode extending along themiddle of the cell midway between the two broader (H-plane) opposing faces. This electrode must be located with great precision since, if it is slightly nearer to one face than the other at any point it will give a greater electric field strength on that side, and any non-uniformity of the field will result in a broadening of the shifted line, and the non-uniformity of the resulting electrostatic forces on the electrode may cause it to oscillate mechanically, the result of which may be an apparent microwave ab sorption signal at the appropriate frequency and phase. Yet at the same time the electrode must be electrically insulated from the guide. Also, the presence of this Stark electrode introduces appreciable attenuation and tends to produce unwanted reflections despite care in its shaping and location. The problem of mounting the electrode in a cell usually a metre or more in length is such that its removal for cleaning is generally a skilled job which may require its return to the manufacturers. This can be a serious drawback where contamination from the gas sample occurs, e.g., due to inadvertent electrical breakdown across the electrode-supporting insulation, which results in a conductive or semiconductive path across the insulation.

It has been proposed by DH. Baird and others (Review of Scientific Instruments Vol 21 No. 10 1950 page 881) to apply the Stark field between two opposite walls of the cell by splitting the cell into two mutually insulated halves, the plane of the split being along the axis of the cell and parallel to the plane of the main electric field (the E-plane). Lide improved on this (Review of Scientific Instruments Vol 35 1964 page 1226) 'by increasing substantially the dimension in the E- plane, so as to improve the uniformity of the Stark field. However a drawback of this cell was its rigid construction, with two halves permanently connected together by strips of insulating material. This was liable to distortion when heated and also was difficult to take apart for cleaning.

The aim of the present invention is to provide an absorption cell of this type, which can be readily taken apart for cleaning purposes but which at the same time is rigid when assembled, and keeps the two halves acc urately in their correct relative positions, as well as being substantially free from distortion with changes in temperature.

According to the invention there is now proposed an absorption cell for use in Stark modulation microwave spectroscopy comprising a rectangular-section waveguide split in a central plane midway between and parallel to the planes of the longer sides of the crosssection to form two mutually insulated halves, the two halves being releasably held spaced apart by a series of releasable clamps spaced apart along their length.

Each clamp is preferably in the form of a C-shapcd member surrounding three sides of the waveguide, with a spring (which may be a leaf spring) spanning the fourth side and bearing on one of the channel-section members to urge it towards the other.

The invention will now be further described by way of example with reference to the accompanying drawings, in which;

FIG. 1 is a simplified block circuit diagram showing a Stark modulation microwave spectrometer;

FIG. 2 is a perspective view of the cell according to the invention; and

FIG. 3 is a cross-section through the cell according to the invention, illustrating the construction of one of the clamps.

Referring first to FIG. 1, a microwave spectrometer operating on the principle of Stark modulation, that is to say, modulation by the Stark effect, comprises basically an oscillator 0, an absorption cell C and a detector D. The oscillator may for example be a Klystron oscillator but is preferably a backward wave oscillator since the latter has a greater ability to be varied in frequency over a range of as much as 1.6 to l purely electrically, without requiring any mechanical movement such as a change of cavity size. In the example under consideration the oscillator operates over the 0 band (called the R band in the USA.) which covers 26,500 to 40,000 Megaherz. The signal from this oscillator is passed through the cell C which may be a metre long, and into which the substance under examination is introduced. Where the substance is a gas it can be fed in at normal temperature, but at a low pressure of the order of 10" Torr. Where it is a liquid at normal temperatures it will normally vaporise anyway at this pressure, but it may be introduced at an elevated tempera ture, and solids may likewise be heated to vaporise them. The frequency of the oscillator is varied slowly over the whole band, or over a part of the band, by a sweep-frequency oscillator S giving a saw-tooth signal of which the sweep period can be varied between 10 seconds and minutes, and as the frequency of the oscillator 0 passes through the frequency of one of the resonant modes of a molecule present in the substance under examination (which may be a mixture) the energy is absorbed to some degree and the signal in the detector D (which is a crystal diode) falls. As the absorption is, in absolute terms, very small (only perhaps one millionth of the total energy passing through the cell, it is impossible to detect directly the fall in signal as the oscillator sweeps through a resonant frequency. Instead a modulation is applied, and in the Starkspectrometer this is applied to the substance itself, making use of the Stark effect by which an electric field applied to the molecules will shift the resonant frequency slightly. The modulation signal is, as far as possible, a square wave signal with as short a rise time as possible, to shift the resonant frequency back and forth between two distinct values. Thus with the oscillator at a given frequency (the sweep being slow and therefore to be ignored in relation to the modulation frequency) which is a resonant frequency for the molecules in the absence of an electric field, a signal will appear in the diode D at the modulation frequency, as the resonant frequency of the molecules shifts between its natural value, to which the oscillator is tuned, and the shifted value. As shown in FIG. 1, the output of the diode D is fed to an amplifier A tuned to the modulation frequency (which may be 40 kiloherz) and used to control the Y deflection of a cathode ray oscilloscope CR or an XY recorder, the X deflection or time-base of which is provided by the saw tooth signal from the sweep oscillator 0.

Additional signals will appear in the diode D as the osciallator 0 passes through frequencies differing by the amount of the modulation frequency from the Stark frequency, and these will appear as so-called Stark lobes on each side of the main Stark signal on the CRO screen or recorder chart. By including a phase-sensitive detector we make these lobes of opposite sign to the main signal, to produce a spectrum (for one particular resonant frequency) of the kind indicated on the CRO screen in FIG. 1.

The absorption cell C according to the invention is shown in FIGS. 2 and 3. A waveguide for the Q band (in U.S.A. the R band), operating in the normal TE mode, would have a cross-section of 0.702 cm. by 0.315 cm., the shorter dimension being that of the opposed walls that lie parallel to the E-plane. To apply the Stark electrostatic field by splitting a cell of such dimensions in a central longitudinal plane midway between these two shorter walls and applying the signal between the resulting two mutually insulated halves would, as indicated earlier, produce unstatisfactory results, since the electrostatic field produced would be non-uniform. The degree of Stark shift of the resonant frequency is proportional to the field strength, which needs to be of the order of two or three thousand volts per centimetre for a worthwhile shift of one Megaherz. If this field strength is not uniform the shift will be spread over a range and result in a loss of resolution. In the cell according to the invention, therefore, we increase substantially the dimension of the E-plane, so that it is greater than the other dimension, that is to say, the width of each of the opposing walls of the cell between which the Stark voltage is applied is greater than the spacing between those walls. In the example shown the width is 2.54 cm., over three times the theoretically correct value. The spacing between them is 0.702 cm., which is the normal dimension in this direction for a waveguide in the Q band, and there is a gently tapering transition section T at each end of the cell to change over the internal dimension in the E-plane (but not the H-plane) from the standard waveguide cross-section to that of the cell.

The two rectangular channel sections F that face each other to form the cell are machined by milling or electro-forming from drawn metal channel sections. They are held in their correct relationship by a series of C-clamps at intervals along their length, two of the clamps being visible in FIG. 2. Each clamp comprises a flat rigid C-shaped member G extending around three sides of the cell and a leaf spring L of flattened V-shape that engages notches in the inner faces of the two ends of the member G. Opposed pairs of B of PTFE of Teeshaped cross-section fit between the flanges of the two channels F to keep them at the correct spacing apart (0.15 mm. in the example shown) and have their outer faces grooved to engage and be located by the inner edges of the member G. The outside face of the web of one of the channels F (the right-hand one on FIGS. 2

and 3) can have shallow transverse grooves machined in it at longitudinally spaced intervals as shown at I in FIG. 3 to receive the inner edges of the webs of the members G of the clamps and thereby locate the clamps axially. The clamps are thus at the electric potential of this channel, normally the grounded channel, and the clamp is insulated from the other channel, by a pad P of PTFE.

The ends of the leaf springs L are themselves notched to locate them in the members G. It will be appreciated that the resulting clamp can be readily assembled and dismantled by hand and that, when assembled, it urges the two halves of the cell tightly and resiliently together. The simple shape of the parts, including the channels F, enables their dimensions to be closely controlled during manufacture, so that the two halves of the channel are accurately and rigidly located in the correct relative positions and minimum spacing (consistent with the voltage to be applied between them) to provide a smooth waveguide offering a minimum of reflections and unwanted modes. At the same time, the simple manual way of releasing the clamps ensures that the user can quickly take them apart to dismantle the cell for cleaning purposes. This is particularly useful in a cell intended for use in routine chemical analysis, where unexpected condensation or decomposition, within the cell, of (possibly unknown) constituents may occur.

The rigid construction resulting from the channels F and the clamps G,L allows the cell to be heated or cooled without significant distortion. The cell will normally be mounted in a close fitting round cylindrical housing to which the gas or vapour under examination is admitted, and the members G, as well as forming parts of the clamps, also form spaced supports for the cell within the housing, which is indicated in broken lines at H in FIG. 3. For this purpose at least the lower limb of each member can, as shown, be of part-circular profile, or at least have as an outer periphery containing points lying on a circle centered on the axis of symmetry of the cell, so that the cell is then located centrally in the housing. We claim:

1. An absorption cell for use in Stark modulation microwave spectroscopy comprising a rectangular section wave guide of cross-section having longer sides and shorter sides and split in a central plane midway between and parallel to the planes of said longer sides to form two channel-section members, opposed blocks of electrically insulating material extending along the length of said wave-guide and having portions of predetermined width projecting between the mutually opposed flanges of said channel section members to mutually space said members the correct distance apart while mutually insulating one from the other, and a series of longitudinally spaced apart releasable clamps each engaging a portion of each of the opposed blocks and releasably holding said blocks and said members in assembled condition.

2. The cell set forth in claim 1 wherein said clamp comprises a C-shaped member having a web and limbs surrounding three sides of said waveguide, said limbs engaging the portions of said blocks, and a spring spanning the fourth side, said spring bearing on one of said channel-section members to urge it towards the other said channel-section member.

3. The cell set forth in claim 2 wherein said spring is a leaf spring.

4. The cell set forth in claim 2 including a pad of electrically insulating material interposed between said spring and said one channel-section member.

5. The cell set forth in claim 3 wherein the free end of at least one of said limbs of the C-shaped member includes a notch 'detachably engaged by said leaf spring.

6. The cell set forth in claim 1 wherein said portions of the blocks engaged by said clamps are grooved to be located by said clamps.

7. The cell set forth in claim 1 wherein each said clamp has a periphery containing points lying on a cirblocks are arranged in longitudinally spaced apart, op-

posed pairs of blocks. 

1. An absorption cell for use in Stark modulation microwave spectroscopy comprising a rectangular section wave guide of cross-section having longer sides and shorter sides and split in a central plane midway between and parallel to the planes of said longer sides to form two channel-section members, opposed blocks of electrically insulating material extending along the length of said wave-guide and having portions of predetermined width projecting between the mutually opposed flanges of said channel section members to mutually space said members the correct distance apart while mutually insulating one from the other, and a series of longitudinally spaced apart releasable clamps each engaging a portion of each of the opposed blocks and releasably holding said blocks and said members in assembled condition.
 2. The cell set forth in claim 1 wherein said clamp comprises a C-shaped member having a web and limbs surrounding three sides of said waveguide, said limbs engaging the portions of said blocks, and a spring spanning the fourth side, said spring bearing on one of said channel-section members to urge it towards the other said channel-section member.
 3. The cell set forth in claim 2 wherein said spring is a leaf spring.
 4. The cell set forth in claim 2 including a pad of electrically insulating material interposed between said spring and said one channel-section member.
 5. The cell set forth in claim 3 wherein the free end of at least one of said limbs of the C-shaped member includes a notch detachably engaged by said leaf spring.
 6. The cell set forth in claim 1 wherein said portions of the blocks engaged by said clamps are grooved to be located by said clamps.
 7. The cell set forth in claim 1 wherein each said clamp has a periphery containing points lying on a circle centered on the longitudinal axis of said waveguide.
 8. The cell set forth in claim 2 wherein the external surface of the web portion of one of said channel-section members carries longitudinally Spaced transverse grooves each capable of being engaged by, and thereby locating, the inner edge of said web of one of said C-shaped members.
 9. The cell set forth in claim 1 wherein the opposed blocks are arranged in longitudinally spaced apart, opposed pairs of blocks. 